Hydrogen metabolism in organisms with oxygenic photosynthesis: hydrogenases as important regulatory devices for a proper redox poising?

Three different hydrogenases and the nitrogenase putatively participate in the hydrogen metabolism of micro-organisms carrying out oxygenic photosynth esis. Hydrogenases either produce hydrogen or split hydrogen into protons a nd electrons depending on their redox partners, whereas the nitrogenase pro duces hydrogen unidirectionally as a byproduct during the reduction of nitr ogen to ammonia. Hydrogenases are well-characterized enzymes on the enzymat ic, structural and genetic level, especially in prokaryotic micro-organisms . They can be classified regarding the metal composition of their active si te (Fe-only, NiFe or metal-free), their preferential direction of reaction (uptake only or bidirectional/reversible) and their in vivo electron donors or accepters. The main physiological role of the uptake hydrogenase in cya nobacteria is probably recapturing the hydrogen produced by nitrogenase. Th e role of the bidirectional hydrogenase in phototrophs is still a matter of debate. Based on recent results which showed it to be of the NAD(P)-reduci ng type, a model for its physiological function is suggested. This model in cludes that this type of hydrogenase is linked to complex I of the respirat ory electron-transport chain and might be an important electron valve durin g photosynthesis under rapidly changing light conditions. The existence of an Fe-only hydrogenase as well as an NiFe-hydrogenase in green algae is sti ll enigmatic and is discussed as hydrogenases either participating in the p roduction of hydrogen or during fermentation. (C) 1998 Elsevier Science S.A . All rights reserved.